1. Field Of The Invention
This invention pertains to the field of orthodontic brackets. More particularly, the present invention is directed to orthodontic brackets being composed of single crystal oxides, such as, a crystalline alpha alumina, zirconia, yttria, magnesia, titania, strontium titanate, and the like, wherein at least a portion of the bracket is provided with an outer protective coating layer of a polycrystalline oxide material. The polycrystalline outer layer improves the fracture toughness of the orthodontic bracket.
2. Discussion Of Related Art
Orthodontic brackets attach directly to teeth and serve to transmit corrective forces from an orthodontic arch wire to the tooth to which the bracket is attached. The requirements for an orthodontic bracket are quite severe. Firstly, it must have sufficient mechanical strength to withstand the forces to which it will be subjected, including the forces transmitted by an arch wire, ligation forces, and mastication forces. Secondly, it must be chemically inert in the oral environment so that it will not corrode and will be and remain biologically inert. The bracket must meet these requirements, and still remain small enough to fit on the tooth.
The overwhelming majority of orthodontic brackets in use today are made of metal, usually stainless steel. Metal brackets meet all of the essential requirements, but they have one undesirable attribute--they are unsightly. A person undergoing orthodontic treatment has a conspicuous amount of metal in full view on the front surfaces of his or her teeth. Since the treatment extends over a number of years, this unsightly appearance must be endured for a considerable period of time.
The incentive to make brackets from less unsightly materials has existed for many years. But recently, orthodontic treatment has been given to increasing numbers of adults, for whom the unsightly appearance of metal brackets is more than a mere annoyance. Ceramic brackets have been proposed but have a tendency to stain, especially if the bracket must be worn for extended periods of time. Consequently, the incentive to provide more esthetic orthodontic treatment is even greater now than it has ever been.
Recently, sapphire (crystalline alpha-alumina) has found commercial application as the material of construction for orthodontic brackets. U.S. Pat. No. 4,639,218 discloses sapphire orthodontic brackets, and sapphire orthodontic brackets are currently manufactured and sold by "A" Company, Inc., a subsidiary of the Johnson & Johnson Company, and Ormco Corporation. While these brackets are esthetically pleasing, i.e., are transparent and do not stain, concerns still exist about the durability of these brackets made from sapphire under the considerable stresses on the bracket induced by the arch wire, ligation and mastication.
Thus, sapphire is more subject to fracturing and chipping than metal. Quite commonly, such fracturing and chipping are initiated by surface damage caused during arch wire installation or by mishandling prior to installation. Due to the small size of these brackets, typically about 1 to 5 millimeters, even a shallow surface defect can be enough to cause a fracture along a crystal lattice in the bracket under the sizable stress of the arch wire. This susceptibility of the crystalline material to fracturing and chipping is clearly undesirable from the standpoint of its use as an orthodontic bracket.
Accordingly, it is desired to provide crystalline orthodontic brackets having enhanced resistance to fracturing and chipping during both installation and use.